1. Field of the Invention
The present invention relates to a distributed element filter used in the RF (radio frequency) stage, etc. for mobile communication equipment as a bandpass filter to suppress noise and interfering signals, and more particularly to a distributed element filter which has flat amplitude characteristics and a flat group delay time in the passband, and transmission zeros in the stopbands, and is simplified in configuration so as to reduce losses for the improvement in performance so as to be advantageously used as a band pass filter.
2. Description of the Related Art
In high frequency circuit sections such as the RF stage of transmitter and receiver circuits for mobile communication system represented by analog or digital portable telephones or wireless telephones are often used bandpass filters (BPFs), for example, to attenuate harmonics radiation which are caused by the nonlinearity in amplifier circuits, or to eliminate undesired signal waves such as interfering waves, sidebands, etc. from the desired signal waves, or when using a common antenna for both the transmitter and the receiver circuits, to separate out the transmitter frequency band and the receiver frequency band that is different from the transmitter frequency band.
Generally, an ideal filter should have characteristics to pass desired signals without producing any distortion and to sufficiently attenuate interfering signals outside the passband. As shown in the diagrams of FIGS. 16A and 16B depicting the filter amplitude and the group delay time, the ideal filter characteristics must have a flat amplitude 17 as well as a flat group delay 18 throughout the passband, while at the same time, realizing attenuation poles 19, 20, i.e., transmission zeros, in the stopbands. In the prior art, complex circuit design has been required for the realization of such a filter.
Techniques for directly realizing a bandpass filter having such characteristics, based on a clear design procedure, are not known in the prior art, and it is common practice to construct filters empirically by mixture of various known techniques.
On the other hand, band pass filters for such communication applications are generally realized and constructed as filter circuits having the desired passband/stopband characteristics by connecting series or parallel resonant circuits constructed with various circuit elements in a plurality of stages. In many cases, filter circuit blocks are constructed by unbalanced distributed constant transmission lines such as coupled microstrip lines or patch resonators, because they have good electrical characteristics for high frequency circuits, and are small in size as circuit elements, and so on.
In fact, using coupled microstrip lines, band pass filters with characteristics having no attenuation poles can be easily realized. Conventional filters composed of a plurality of coupled resonators by quarter wavelength xcex/4 (xcex is the wavelength) coupled microstrip lines have uniformized coupling structure and generally allow little freedom in design, for example the sign, positive or negative, of each coupling reactance element cannot be chosen freely as described hereinafter. Consider the prior art example shown in FIGS. 17A and 17B. A ladder network with parallel and series resonators in FIG. 17A is transformed using imaginary gyrators to a circuit in FIG. 18 which is compose of only parallel resonators that are easy to realize. FIG. 17A is an original circuit an example of a third order filter, and FIG. 17B is its strictly equivalent circuit that is derived using imaginary gyrators 21, 22.
In this case, for a strict transformation from the filter of FIG. 17A to the equivalent filter of FIG. 17B, the two imaginary gyrators 21, 22 must be made opposite in sign. That is, strictly, the coupling reactance elements must have both signs, positive and negative. In practice, it is difficult to realize a coupling structure by xcex/4 coupled microstrip lines to achieve this.
On the other hand, in the case of a filter with simple characteristics having no attenuation poles, since no cross coupling is required in the filter circuit, there is no need to strictly control the positive and negative signs of the coupling; consequently, the imaginary gyrators may have only the positive or the negative sign, or the positive and the negative signs may be interchanged. As a result, the filter circuit can be realized without any problem, even with a structure in which a plurality of resonators formed by xcex/4 coupled microstrip lines are sequentially coupled in the same manner.
By contrast, in the case of a filter with complex characteristics that have attenuation poles or that need controlling the group delay and amplitude characteristics, a cross coupling structure is needed in the filter circuits, and the positive and negative phases of the coupling characteristics must be controlled strictly. As a result, xcex/4 coupled microstrip lines cannot arbitrarily give the positive and negative phases of the coupling characteristics, and it is difficult to use them as circuit elements for a filter circuit, and hence, it is difficult to create desired attenuation poles or to get prescribed amplitude and group delay characteristics by filter elements by xcex/4 coupled microstrip lines.
Multi-resonator filters constructed by connecting such xcex/4 coupled microstrip lines in multiple stages usually use straight microstrip lines; on the other hand, so-called hairpin-type multi-resonator filters constructed with microstrip resonators formed from bent microstrip lines called hairpin transmission lines are also used. Examples are shown in FIGS. 18A and 18B; FIG. 18A is a plan view showing an example of a multi-resonator filter of straight line type constructed by sequentially coupling four microstrip resonators 1 to 4 formed from straight microstrip lines, and FIG. 18B is a plan view showing an example of a multi-resonator filter of hairpin type constructed by sequentially coupling four microstrip resonators 5 to 8 formed from hairpin microstrip lines.
The hairpin-type multi-resonator filter, however, has the same problem as described above.
To solve the above problem, the inventor has previously proposed distributed element filters constructed with microstrip resonators in multiple stages formed by sequentially cascading quarter wavelength of the center frequency of the passband coupled microstrip lines. In these distributed element filters, a resonator sequentially coupling method that allows to align accurately the phase of the transmission characteristics is employed assuming by adding a cross coupling circuits to the sequentially coupled resonators it becomes possible to form attenuation poles and control the amplitude characteristic as well as group delay time.
However, the problem to he solved with these distributed element filters is how the cross coupling circuit is connected to the sequentially coupled microstrip resonators formed on the same plane. More specifically, when forming a cross coupling circuit for realizing the desired characteristics, and when the number of resonators to be cross coupled is an even number equal to or more than 4; as a result, if the cross coupling is to be made among the quarter wavelength coupled microstrip lines or quarter wavelength coupled hairpin microstrip lines formed on the same plane, as shown, for example, in the plan views of FIGS. 19A to 19D depicting a filter configuration example, the cross coupling circuit 11, 14 inconveniently has to cross the resonator pattern indicated at 1 to 10, 12, 13. It is therefore required that the cross coupling circuit be formed in a three dimensional structure; that is, in an air bridge structure, for example, through the space over the resonator pattern. This, in turn, leads to the drawback that the advantage that this distributed element filter is a planar circuit is lost.
In the filters shown in FIGS. 19A and 19B, of the four coupled straight microstrip resonators the first and fourth resonators 1, 4 are connected to the cross coupling circuit 11 directly (FIG. 19B) or via external circuit connection lines 9, 10 (FIG. 19A) coupled to the respective resonators. Likewise, in the filters shown in FIGS. 19C and 19D, of the four coupled hairpin microstrip resonators the first and fourth resonators 5, 8 are connected to the cross coupling circuit 14 directly (FIG. 19D) or via external circuit connection lines 12, 13 (FIG. 19C) coupled to the respective resonators. In any of these examples, the cross coupling circuit 11, 14 crosses one of the first to fourth resonators 1 to 4; 5 to 8, leading to the problem that the cross coupling circuit needs to be formed in a three dimensional structure in order to prevent the formation of an electrical connection at this crossing point.
In this way, when connecting a cross coupling circuit 11, 14 to a distributed element filter constructed with an even number (equal to or more than 4) of sequentially coupled and connected microstrip resonators formed on the same plane, the cross coupling circuit must be formed in a three dimensional structure to prevent it from shunted to any one of the resonators 1 to 4; 5 to 8. It is therefore desired to achieve cross coupling of the design value in a distributed element filter by using only a two dimensional structure. This would enable attenuation poles to be formed and the amplitude characteristic and group delay time to be adjusted within a filter of a simple planar structure, offering an enormous practical advantage that a band pass filter that has band pass characteristics achieving both a flat amplitude and a flat group delay over the passband, while at the same time, realizing transmission zeros in the stopbands, could be constructed and realized with simple circuitry supported by an accurate design technique. It is thus desired to realize the connection of a cross coupling circuit on the same plane without employing a three dimensional structure, and thereby provide a distributed element filter that can be realized and fabricated easily without impairing the advantage of the distributed element filter of the planar structure.
The invention has been devised to solve the above-outlined problem, and its object is to provide a distributed element filter that has band pass characteristics achieving both a flat amplitude and a flat group delay over the passband, while at the same time, realizing transmission zeros in the stopbands, by realizing the connection of a cross coupling circuit on the same plane without employing a three dimensional structure and without impairing the advantage of the distributed element filter of the planar structure, and that has low sensitivity and low loss characteristics and is capable of being constructed and realized with simple circuitry supported by an accurate design technique.
The distributed element filter of the invention is based on a distributed element filter with band pass characteristics, realized by an unbalanced distributed constant circuit and obtained by a frequency transform from a low pass prototype filter whose transfer function is expressed by a circuit network function consisting of a numerator rational polynomial, which is an even function of complex frequency s and has a pair of plus and minus real zeros or a pair of conjugate purely imaginary zeros, and a denominator rational polynomial, which is a Hurwitz polynomial of the complex frequency s.
As shown in FIGS. 1A to 1D given later, the invention provides a distributed element filter comprising:
n half wavelength of a passband center frequency, microstrip resonators (L, H) consisting of straight and hairpin microstrip lines (L, H) wherein n is an even number equal to or more than 4, the n half wavelength microstrip resonators (L, H) being connected sequentially with each resonator coupled with adjacent resonators over a length of approximately one quarter wavelength, respective numbers of straight microstrip lines (L) and hairpin microstrip lines (H) of the n half wavelength microstrip resonators (L, H) being both odd;
an external circuit connection quarter wavelength straight microstrip lines (M) coupled to first and n-th half wavelength microstrip resonators (L1, L4; H1, H4), respectively; and
a cross coupling circuit (C) connected to ends of the first and n-th half microstrip resonators (L1, L4; H1, H4), the ends being of a side on which the first and n-th half microstrip resonators (L1, L4; H1, H4) are coupled with the external circuit connection quarter wavelength straight microstrip lines (M) (FIGS. 1B, 1D), or to ends of the external circuit connection quarter wavelength straight microstrip lines (M) (FIGS. 1A, 1C).
According to the distributed element filter of the invention, the n half wavelength straight or hairpin microstrip resonators are connected sequentially with each resonator coupled with adjacent resonators over a length of approximately one quarter wavelength, the number of straight microstrip lines and the number of hairpin microstrip lines both being set odd, while the external circuit connection straight microstrip lines, each having approximately one quarter wavelength, are coupled to the first and n-th half wavelength microstrip resonators, respectively, and the cross coupling circuit is connected to the ends of the first and n-th half microstrip resonators, which ends are of a side on which the first and n-th half microstrip resonators are coupled with the external circuit connection quarter wavelength straight microstrip, or to the ends of the external circuit connection quarter wavelength straight microstrip lines. This enables the cross coupling circuit to be connected on the same plane without using a three dimensional structure, and the zeros of the numerator rational polynomial, that is, transmission zeros can be realized as transmission zeros of the transmission characteristics of the filter.
Furthermore, by adding an electric field or magnetic field cross coupling circuit to a multi-resonator band pass filter constructed with n resonators, it becomes possible to form desired attenuation poles and to adjust the amplitude characteristic and group delay time. Moreover, by using the cross coupling circuit to control the phase of the transmission characteristic between the resonators, desired attenuation poles can be formed and the amplitude characteristic and group delay time adjusted using only the cross coupling circuit of nearly the same type, which facilitates the realization of a distributed element filter having the desired characteristics.
Further, when n is 6 or larger, the cross coupling can be implemented in the form of multiple cross coupling such as double or triple, or even in the form of a cascade connection of a plurality of multi-resonator filters including the cross coupling.
As a result, a distributed element filter can be provided that has band pass characteristics achieving, without impairing the advantage of the distributed element filter of the planar structure, both a flat amplitude and a flat group delay over the passband, while at the same time, realizing transmission zeros in the stopbands, and that has low sensitivity and low loss characteristics and is capable of being constructed and realized with simple circuitry supported an accurate design technique.
In the invention it is preferable that, as shown in FIGS. 2A to 2D, at least one of the half wavelength microstrip resonators is replaced by a one-wavelength microstrip resonator (H11).
According to the distributed element filter of the invention, since replacing at least one of the half wavelength microstrip resonators by a one-wavelength microstrip resonator in the above configuration achieves the effect of reversing the phase of the transmission characteristics of the multi-resonator filter in a controlled manner, the cross coupling circuit can be added exactly as intended by the design.
According to the distributed element filter of this invention, since, in design theory, the circuit block corresponding to the real zeros or imaginary zeros of the numerator rational polynomial of the circuit network function describing the transfer characteristic is implemented by the multi-resonator filter of the above configuration, a filter circuit that is theoretically accurate and is simple in structure, and that provides improved performance allowing low losses and has the desired filter characteristics, can be constructed and realized using distributed constant elements on the same plane without using a three dimensional structure.
The transmission zero corresponding to zero on the imaginary axis of the transfer function can be realized by applying cross couplings to the coupling/connection between the resonators, and the amplitude by zeros on the real axis of the transfer function can be modified. The zero on the imaginary axis and the zero on the real axis can be realized by the cross coupling circuits of nearly the same structure. Consequently the phase of the transmission characteristics can be easily controlled. As a result, a band pass filter having characteristics that achieve both a flat amplitude and a flat group delay over the passband, and that realizes transmission zeros (attenuation poles) in the stopbands, can be realized with simple circuitry.
In the invention it is preferable that the distributed element filter has band pass characteristics in which both of amplitude characteristics and group delay characteristics of the passband are flat and a transmission zero is in a stopband thereof.
As described above, according to the invention, a distributed element filter can be provided that has band pass characteristics achieving, without impairing the advantage of the distributed element filter of the planar structure, both the flat amplitude and the flat group delay over the passband, while at the same time, realizing transmission zeros in the stopband, and that has low sensitivity and low loss characteristics and is capable of being constructed and realized with simple circuitry supported by an accurate design technique.
As shown in FIGS. 12A to 15 given later, the invention provides a distributed element filter comprising:
n half wavelength corresponding to a passband center frequency, microstrip resonators (L, H) consisting of straight and hairpin microstrip lines (L, H) wherein n is an even number equal to or more than 4, the n half wavelength microstrip resonators (L, H) being connected sequentially with each resonator coupled with adjacent resonators over a length of approximately one quarter wavelength, respective numbers of straight microstrip lines (L) and hairpin microstrip lines (H) of the n half wavelength microstrip resonators (L, H) being both odd;
an external circuit connection quarter wavelength straight microstrip lines (M) coupled to first and n-th half wavelength microstrip resonators (L1, L4; H1, H4; H1a, H4a; H1b, H4b), respectively; and
a cross coupling circuit (C, C1, C1a, C1b,) consisting of an a/2 wavelength microstrip line (u1, u3, u5, u7, u9, u11, u3a, u9b) and a b/2 wavelength microstrip line (u2, a4, u6, u8, u10, u12, u4a, u10b) capacitively coupled via a slit (g1, g2, g1a, g1b) (a and b are natural numbers), the cross coupling circuit (C) being connected to ends of the first and n-th half microstrip resonators (L1, L4; H1, H4), the ends being of a side on which the first and n-th half microstrip resonators (L1, L4; H1, H4) are coupled with the external circuit connection quarter wavelength straight microstrip lines (M), or to ends of the external circuit connection quarter wavelength straight microstrip lines (M1, M4, M1a, M4a, M1b, M4b).
According to the distributed element filter of the invention, the n half wavelength straight or hair pin microstrip resonators are connected sequentially with each resonator coupled with adjacent resonators over a length of approximately one quarter wavelength, the number of straight microstrip lines and the number of hairpin microstrip lines both being set odd, while the external circuit connection straight microstrip lines, each having approximately one quarter wavelength, are coupled to the first and n-th half wavelength microstrip resonators, respectively, and the cross coupling circuit consisting of an a/2 wavelength microstrip line and a b/2 wavelength microstrip line capacitively coupled via a slit (a and b are natural numbers) is connected to the ends of the first and n-th half microstrip resonators, which ends are of a side on which the first and n-th half microstrip resonators are coupled with the external circuit connection quarter wavelength straight microstrip, or to the ends of the external circuit connection quarter wavelength straight microstrip lines. This enables the cross coupling circuit to be formed on the same plane without using a three dimensional structure, and the zeros of the numerator rational polynomial, that is, real transmission zeros or imaginary transmission zeros can be realized.
Furthermore, by adding an electric field or magnetic field cross coupling circuit to a sequential multi-resonator band pass filter constructed with n resonators, it becomes possible to form desired attenuation poles and to adjust the amplitude characteristic and group delay times. Moreover, by using the similar cross coupling circuit to adjust the phase of the transmission characteristic between the resonators, desired attenuation poles can be also formed. The amplitude characteristic and group delay time can be therefore adjusted using the cross coupling circuits of nearly the same structure, which facilitates the realization of a distributed element filter having the desired characteristics.
Further, when n is 6 or larger, the cross coupling can be also implemented in the form of multiple cross coupling such as double or triple, while the cross coupling is implemented also in the form of a cascade connection of a plurality of multi-resonator filters including a single cross coupling.
As a result, a distributed element filter can be provided that has band pass characteristics achieving, without impairing the advantage of the distributed element filter of the planar structure, both a flat amplitude and a flat group delay over the passband, while at the same time, realizing transmission zeros in the stopbands, and that has low sensitivity and low loss characteristics and is capable of being constructed and realized with simple circuitry supported by an accurate design technique.
In the invention it is preferable that as shown in FIG. 14, in the plurality of distributed element filters (71, 72) the external circuit connection quarter wavelength straight microstrip lines (M1, M4, M1a, M4a) are connected in cascade, and at least one of the respective values of (a+b) for the microstrip line (M1, M4, M1a, M4a) in the cross coupling circuit (C1, C1a) of the plurality of distributed element filters (71, 72) is odd and at least one thereof is even.
According to the distributed element filter of this invention, a plurality of distributed element filters according to the invention are connected as filter blocks in cascade; when these filter blocks are identical in configuration, since the value of (a+b) for the cross coupling circuit is chosen to be odd in one filter block and even in another filter block, the respective cross coupling circuits become equivalent to electric field coupling and magnetic field coupling or magnetic field coupling and electric field coupling, and it follows that the filter blocks having complementary cross coupling are connected in cascade.
In the invention it is preferable that, as shown in FIG. 15, in a plurality of the distributed element filters (73, 74) the external circuit connection quarter wavelength straight microstrip lines (M1, M4, M1b, M4b) are connected in cascade, and at least one of the half wavelength microstrip resonators is replaced by a one-wavelength microstrip resonator (H21b).
According to the distributed element filter of this invention, in a plurality of the distributed element filters according to the invention, the external circuit connection quarter wavelength straight microstrip lines are connected in cascade, and at least one of the half wavelength microstrip resonators is replaced by a one-wavelength microstrip resonator. Accordingly, addition of the same type of cross coupling circuits makes it possible to form desired attenuation poles and adjust the amplitude characteristics and group delay times with the result that a distributed element filter having desired characteristics can be realized. Since this makes it possible to form attenuation poles, or to flatten the amplitude or/and to adjust the group delay times by using the cross coupling circuit in each filter block, thus flattening the amplitude and group delay characteristics over the passband while realizing attenuation poles in the stopbands, the distributed element filter thus constructed can, as a whole, achieve the desired passband as well as stopband characteristics.
In the invention it is preferable that the distributed element filter has band pass characteristics in which both of amplitude characteristics and group delay characteristics of the passband are flat and transmission zeros are formed in a stopband thereof.
According to the distributed element filter of this invention, since, in design theory, the circuit block corresponding to the real zeros or imaginary zeros of the numerator rational polynomial of the circuit network function describing the transfer characteristic is implemented by the multi-resonator filter of the above configuration, a filter circuit that is theoretically accurate and is simple in structure, and that provides improved performance allowing low losses and has the desired filter characteristics, can be constructed and realized using distributed constant elements on the same plane without using a three dimensional structure.
As described above, according to the invention, transmission zeros corresponding to zeros on the imaginary axis of the transfer function can be realized by forming a cross coupling circuit for the coupling/connection between the microstrip resonators, and the amplitude can be adjusted corresponding to zeros on the real axis of the transfer function. In connection with this adjustment, the phase of the transfer characteristic can also be easily controlled. As a result, a distributed element filter having both a flat amplitude characteristics and a flat group delay characteristics over the passband and transmission zeros (attenuation poles) in the stopbands, can be realized with simple circuitry.
Further, according to the invention, a distributed element filter can be provided that has band pass characteristics achieving, without impairing the advantage of the distributed element filter of the planar structure, both a flat amplitude and a flat group delay over the passband, while at the same time, realizing transmission zeros in the stopbands, and that has low sensitivity and low loss characteristics and is capable of being constructed and realized with simple circuitry supported by an accurate design technique.